The operating principle of a vibratory gyrometer is based on the sensing of Coriolis accelerations acting on an element vibrating according to a first useful mode of vibration, referred to as drive mode, when this element is subjected to a rotation with angular velocity Ω referenced with respect to an inertial frame of reference, referred to as Galilean frame of reference. The Coriolis accelerations are alternating at the frequency of the first useful mode and excite a second transversal useful mode of vibration, referred to as a sense mode and the vibration amplitude of which is proportional to Ω. The vibration amplitude is generally converted into the shape of electric signals and measured to determine Ω.
It is known (EP-A2-1 962 055; EP-A2-0 844 461) that the vibrating element can be a tuning fork formed of two identical and parallel branches facing each other and each fixed each at one end to a common part, as shown in FIG. 1. The drive mode is a flexional resonance vibration of the two branches in phase opposition therebetween parallel to the plane XY of the tuning fork. When the tuning fork is subjected to rotation about an axis parallel to the longitudinal axis of the tuning fork, the sense mode is a flexional resonance vibration of the two branches in phase opposition therebetween perpendicularly to the plane XY of the tuning fork.
The tuning fork is made through chemical machining of a quartz plate of a uniform thickness in an etching bath comprising hydrofluoric acid, as it is implemented for clock resonators. The tuning fork first takes advantage of the excellent dimensional definition of the photolithographic methods, secondly, of the easy piezoelectric excitation and sensing of both useful modes by means of electrodes deposited on the quartz, and third, of a very reduced industrial manufacture cost.
The quartz plate with a uniform thickness is parallel to the electric and mechanical crystallographic plane XY of the quartz to facilitate the chemical machining. The branches of the tuning fork are oriented following a mechanical crystallographic axis Y to allow for a good efficiency of the piezoelectric excitation and sensing.
As a result of the advantages thereof the chemical machining of the quartz inevitably results in oblique facets occurring with respect to the main crystallographic axes X and Y of the tuning fork, as shown in a simplified way on FIG. 1. First oblique facets fo are located at the foot of the branches of the tuning fork, at the start of the common part, and second oblique facets shaped as dihedron fd are located on each of the branches along a machined flank, the other machined flank being nearly planar and perpendicular to the plane of the plate. These facets, having quite large dimensions, are very well defined and reproducible from one machining to another, and do not have any drawback when the tuning fork operates according to a single vibratory mode, as it is the case with clock resonators, as they can be easily taken into account for the geometric definition of photolithographic tools.
In contrast, when the tuning fork is used as a vibrating element of a gyrometer operating according to two useful vibration modes, the oblique facets have the drawback of creating a mechanical coupling between these useful vibration modes within each beam forming a branch of the tuning fork. Such a mechanical coupling results on the sense electrodes in the occurrence of an electric signal that is in quadrature with the Coriolis signal and that should be removed so as to be able to determine the angular velocity Ω.
Two means are known for removing such a quadrature signal.
The first known means implements electronic circuits for signal processing aiming at removing the quadrature signal through synchronous demodulation. The first means is however unsatisfactory when low rotational velocities are to be measured, as the Coriolis signal is then very small with respect to the quadrature signal and it becomes difficult to control with a sufficient accuracy the phase of the electric signals for taking advantage of the synchronous demodulation.
The second known means for removing the quadrature signal consists in directly acting on the vibrating element, for instance carrying a trimming using a mechanical tool until obtaining a sufficient reduction of the electric quadrature signal present on the sense electrodes, as disclosed in U.S. Pat. No. 6,101,878. The second means significantly increases the measurement accuracy for the gyrometer, but has the drawback of increasing the manufacturing cost, as each gyrometer copy being manufactured should be submitted to a specific trimming operation.
In order to reduce the mechanical coupling without carrying out a trimming, it seems interesting to orient the branches of the tuning fork according to another axis than the axis Y in the plane XY of the plate, for example following an electric crystallographic axis X, as shown on FIG. 2. Indeed, the chemical machining of a thus oriented tuning fork produces significantly reduced oblique facets at the foot of the branches and does not produce any oblique facet along the machined flanks of the branches. The mechanical coupling achieved within each beam of such a tuning fork is more significant than the previous embodiment with the branches oriented following the axis Y. This is due to the mechanical anisotropy of the quartz which, for branches oriented following the axis X, prevents that the vibrations of the two useful modes are independent from each other.